Harold Hall

Workshop Processes

First, set up the fence on the milling machine and centre drill using the milling
machine's dials to space accurately the holes into which pin "Z" will fit, that correspond
to the pitch of the scroll. Return the first hole and this time drill with the chosen
hole size. For ease of eventually positioning the fence for use drill more holes
than teeth to be made, say five extra.

With that done, make three stop pins (Z), a close fit in the holes in the fence and
with three sizes of heads. The purpose of this is to ensure the jaws accommodate
the difference in scroll diameter at each jaw position. The diameters of the pin's
heads are relatively unimportant but they must increase by 2/3rd of the scroll's
pitch between plug 1 and 2, and between 2 and 3. Actually, when I parted the pins
off from the material in the chuck the pin was left with a small spigot, as is often
the case when parting off. This I found very useful as it enabled the pin to be held
with a pair of long nose pliers when moving from hole to hole. You may like to use
my accidental method or deliberately machine them with a small spigot. The spigot
is easier to grip than the head of the pin itself which in any case could become
damaged resulting in pitch errors.

Next stage is to mount the fence on the rotary table and this on the milling machine.
The essential requirement is that the fence is set off centre by the offset value
(CP Sk.8) and that the milling machine spindle is exactly above the centre of the
rotary table. To achieve that, first make a plug a close fit in the rotary tables
bore and a head diameter equal to twice the distance of the crest from the edge of
the jaw (2 x CP), also include a centre drilled impression, Sk. 10. It is essential
that the two diameters and the drilled centre are all concentric so they must be
machined at the same time. The smaller diameter will therefore have to be turned
using a left hand knife tool. Also, make another plug to suit the second offset.
Having said that, in my case there was only 0.2mm difference between the two offsets.
This I felt was of little consequence so took the mid position and made just one
plug. If you have a rotary table with a taper bore, you will need to make an adaptor,
see my pages on using a rotary table, which also see if you need help in using this
rather specialised piece of equipment.

Fit the plug, and with a centre mounted in the machines spindle lower this into the
centre drilled plug and fasten the rotary table onto the machine table at that position.
On the basis that you are machining the outer diameter of each jaw tooth, traverse
the table towards you by an amount equal to the required radius plus half the cutter's
diameter. When machining the inner curve it will be the radius required minus half
the cutter's diameter. Do remember to take out the backlash in the appropriate direction
before removing the centre in the spindle from that in the plug before setting on
the radius for machining to take place.